When Gary Wittert began looking for tubby male baby boomers to take part in a clinical trial last month, he got 800 volunteers in one day. The draw: free testosterone injections. Wittert, a professor of medicine at the University of Adelaide, and his colleagues suspect the sex hormone known to increase libido and musculature could also play a role in preventing a form of diabetes that tends to strike later in life and afflicts more than 330 million people worldwide.